sqlalchemy: fix lazy-loading of attributes
sqlalchemy: fix lazy-loading of attributes

Use a string rather than a class attribute to be sure we correctly load
the resource attribute. Not sure it's a good fix but let's try!

Change-Id: I22df441670a1e355e3e1d0e10b2b2ff96d55e5e4
Closes-Bug: #1470940

gnocchixyz/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,sileht/gnocchi,leandroreox/gnocchi,leandroreox/gnocchi